A unit fraction has numerator 1, as in . An Egyptian fraction is a sum of unit fractions, usually distinct. For example, .
Erdös and Straus conjectured that always has a solution over integers for .
Sierpinski conjectured that always has a solution over integers for .
All have been verified for the Erdös–Straus conjecture , but both are still unsolved problems. The Rhind Papyrus and the Egyptian leather scroll solve many similar problems, so these can be considered among the oldest unsolved mathematical problems.